We will test the hypotheses that (1) drug-specific monoclonal antibody Fab' fragment can prevent lethality from an overdose of the tricyclic antidepressant (TCA) desipramine (DMI) in the rat, and that (2) the efficacy and safety of this therapy can be enhanced by the purposeful design of a smaller recombinant antibody fragment. The use of drug- specific antibodies has previously been limited to a single drug (digoxin) that has an unusually low toxic dose and therefore requires a very low dose of antibody to reduce its toxicity. Our ongoing studies have shown that anti-TCA Fab' can reverse sublethal DMI toxicity, demonstrating for the first time that this approach can also be used to reverse the toxicity of drugs that have a large toxic dose. This finding substantially broadens the potential therapeutic role of drug-specific antibodies. In Part 1 we will focus upon obtaining a better understanding of the pharmacokinetic and pharmacodynamic mechanisms by which anti-TCA Fab' reverses DMI toxicity in the rat. We will study whether anti-TCA Fab' prevents lethality from DMI, whether DMI toxicity recurs after initial reversal, the pharmacokinetic correlates of any such recurrence, and the effects of anti-TCA Fab' on DMI elimination. These data should be helpful in optimizing therapy for DMI toxicity, and for anticipating which other drug toxicities might benefit from antibody therapy. In Part II we will construct and evaluate a single chain antibody fragment (SCA) one half the size of Fab. Using a system specifically designed for cloning and expressing Fab fragments, we have produced a recombinant fragment from anti-TCA that binds DMI in vitro. We will confirm its presumed identity as Fab and its ability to reverse DMI cardiotoxicity in rats, and use the genes coding for this fragment to construct SCA. The efficacy of SCA for reversal of DMI toxicity, its pharmacokinetics, and its toxicity will be studied in rats. We postulate that SCA will reverse DMI toxicity more rapidly than Fab and be less toxic owing to its smaller size, larger volume of distribution and more rapid renal elimination. SCA is intended to serve as a prototype for the future development of other antibody fragments that may have therapeutic advantages over Fab. Parts I and II therefore serve as complementary approaches to improving therapy for DMI toxicity, and for expanding the therapeutic uses of drug- specific antibodies.